Coated-type magnetic recording media prepared by coating a magnetic coating composition having ferromagnetic particles and an organic binder onto a non-magnetic support and drying it have widely been used. However, these magnetic recording media have a low saturation magnetization because the metal oxide particles are mainly used as ferromagnetic particles and the density of ferromagnetic particles contained in the magnetic layer can not be increased because of the amount of organic binder contained therein. Also, the manufacturing steps are complicated. Therefore, these types of recording media are not suitable for high density recording.
With the increased demand for high density recording in recent years, a magnetic recording medium comprising a thin metal film on a non-magnetic support has been developed. The above magnetic recording medium is prepared by forming a thin metal film onto a non-magnetic support by a vapor deposition method such as vacuum deposition, sputtering or ion plating, or a plating method such as electroplating or electroless plating.
The magnetic composition is not limited to metals, but metals are typical examples of magnetic compositions. Therefore, the above magnetic recording medium is alternatively referred to as a magnetic recording medium of a thin metal film type, or is called a magnetic recording medium of a non-binder type because such a magnetic recording medium does not contain an organic binder.
The magnetic recording medium of a thin metal film type has recently drawn much attention. Since the thin metal film is made of ferromagnetic metals having high saturation magnetization and does not contain a binder, a higher coercive force can be obtained, the thickness of the film can be reduced and the influence of demagnetization at the short wave length region is slight. Thus, high density recording can be realized and manufacturing steps are simplified.
The thin metal film of the magnetic recording medium appears to have a uniform and smooth metal surface; however, microscopically it has such a rough structure that the metal particles align and the film easily corrodes. Thus, the magnetic recording medium of the thin metal film type has poorer weather resistance and corrosion resistance than the magnetic recording medium of a coated type. The surface of the magnetic recording medium used particularly for a cassette tape and a video tape is rubbed by a magnetic head upon recording and reproducing. When even a slight corrosion is present on the surface of a thin metal film, it drops off upon friction, causing head clogging, whereby both a head and a magnetic recording medium are damaged.
The magnetic recording medium of a thin metal film type also has poor durability. Due to the smoothness of the thin metal film, friction is large and stick-slip readily takes place. Still durability of the medium on a VTR is also poorer than the recording medium of a coated type.
In order to improve weather resistance and durability of the magnetic recording medium of the thin metal film type, a method of providing nitrogenation treatment by ion plating (Japanese Patent Application (OPI) No. 33806/75), a method of providing a silicon nitride film by sputtering (Japanese Patent Application (OPI) No. 30304/78), a method of forming a non-magnetic surface layer by exposing a magnetic film to the discharge in the atmosphere of nitrogen gas (Japanese Patent Application (OPI) No. 85403/78) and a method of providing a nitrogenated thin metal film on a thin magnetic metal film (Japanese Patent Application (OPI) No. 143111/79) have been developed. The term "OPI" as used herein means an unexamined published application.
A thin magnetic film composed of iron nitride or of iron and iron nitride is known as a magnetic recording medium of a non-binder type having excellent weather resistance, as disclosed in European Pat. No. 8,328 and Japanese Patent Application (OPI) No. 87809/84. One of the present inventors, with others, earlier proposed a magnetic recording medium comprising a non-magnetic support having provided thereon a thin magnetic film mainly composed of iron oxide nitride (Japanese Patent Application (OPI) No. 54023/86). The iron oxide nitride has a composition as represented by formula (I) EQU Fe.sub.1-x-y N.sub.x O.sub.y (0.25.ltoreq.x+y&lt;0.60) (I)
In accordance with the above-described method, weather resistance is greatly improved and durability is also improved to some extent by providing an iron oxide nitride layer. However, there are still problems in that scratch resistance at a low humidity is poor and the magnetic layer is fragile.